This invention relates to the art of removing pollutants from the exhaust of combustion systems, in particular, internal combustion engines. Specifically, the invention discloses a device and method for heating a cellular or honeycomb structure used for emission control in order to initiate faster catalytic light-off and/or to heat a diesel particulate filter. More particularly, the invention relates to a filter for removing particulates such as carbon, from the exhaust of diesel engines. The invention also relates to a device, and method for reducing the catalytic light-off time of a cellular or honeycomb structure when such structure is used as a catalytic converter for internal combustion engines.
When a catalytic substrate or structure is heated to a sufficiently high temperature, a reaction occurs which converts the dangerous pollutants to harmless gases. Typically, there is a time span between the time the emission begins, that is, cold start, to the time the substrate heats up sufficiently for catalyst "light-off" to occur. Light-off time is defined as the time it takes to achieve a 50 percent conversion efficiency. The temperature at which light-off occurs is generally in the range of about 250 to 350 degrees centigrade. The unreacted pollutants generated prior to light-off contribute significantly to the total pollutants generated during the start-up cycle of an engine. Modifications are being made to catalytic converters to increase the rate of heating in order to obtain shorter light-off times. By reducing the light-off time, the amount of pollutants escaping into the atmosphere can be reduced.
Diesel particulate filters have been found effective for trapping carbon particles in diesel engine exhaust fumes. For example, U.S. Pat. No. 4,329,162 issued to Pitcher, Jr., herein incorporated by reference, discloses a diesel particulate filter wherein the walls of the individual passageways through the filter are porous, with both the inlet and the outlet of the filter having alternate openings and plugs, as in the manner of the design of a checkerboard. As shown for example at FIG. 2 of the Pitcher patent, the flow is such that approximately one-half of the inlet face area of the filter is defined by open channels which are closed at the exit face of the filter. Because of this closure, the exhaust fumes or gases passing into the inlet of the filter must exit by diffusing through the porous walls between individual passageways. While exhibiting utility for filtering fluid streams, such as filtering carbon particulates from the exhaust gas of diesel engines, filters of the Pitcher type become clogged with use, with attendant increase in back pressure, causing loss of engine efficiency. Regeneration of a clogged filter (removal of the particles lodged therein) usually involve removing the filter from its mounting in an exhaust conduit.
Various methods have been proposed for the regeneration of filters for continuous use, for example, the use of a heater, located ahead of, and separate from the filter as disclosed in U.S. Pat. Nos. 4,878,928, 4,535,589 and 4,211,075. The in situ regeneration of a filter, utilizing the heating effect of electrical currents to incinerate accumulated particulates, is known. However, no one of such filters has been constructed of a porous metal honeycomb or monolith. Also, it is not known to preheat a honeycomb structure for light-off purposes by attaching or connecting electrodes to the structure.
It is therefore the object of the present invention to provide a heated device or structure which can be used to regenerate particulate filters, and/or to provide early light-off of catalytic converters, by energizing electrodes which are connected directly and integral to the surface of the structure.